http://www.gdem.aster.ersdac.or.jp/search.jsp
I downloaded two tiles on monday (17th), but since tuesday I'm unable to download them, maybe because there are too many people downloading them at the moment.
He didn't ask about that kind of radar. He wants to know about GPR, a geophysical method.
Can't answer your question though. Try talking to a geophysicist, or an environmental geologist.